Process for coating an object with silicon carbide

ABSTRACT

A process for coating a carbon or graphite object with silicon carbide by contacting it with silicon liquid and vapor over various lengths of contact time. In the process, a stream of silicon-containing precursor material in gaseous phase below the decomposition temperature of said gas and a co-reactant, carrier or diluent gas such as hydrogen is passed through a hole within a high emissivity, thin, insulating septum into a reaction chamber above the melting point of silicon. The thin septum has one face below the decomposition temperature of the gas and an opposite face exposed to the reaction chamber. The precursor gas is decomposed directly to silicon in the reaction chamber. A stream of any decomposition gas and any unreacted precursor gas from said reaction chamber is removed. The object within the reaction chamber is then contacted with silicon, and recovered after it has been coated with silicon carbide.

ORIGIN OF THE INVENTION

This invention described herein was made in the performance of workunder NASA Contract No. NAS7-100 and is subject to the provisions ofSection 305 of the National Aeronautics and Space Act of 1958 (72 Stat.435; 42 U.S.C. 2457).

CROSS-REFERENCE

This is a continuation of copending application Ser. No. 932,029, filedon Nov. 18, 1986, now abandoned, which is a continuation-in-part ofApplication Ser. No. 749,661 filed June 28, 1985, now U.S. Pat. No.4,668,493, and a continuation-in-part of application Ser. No. 618,712filed June 8, 1984, now U.S. Pat. No. 4,737,348. Application Ser. No.749,661 is in turn a continuation-in-part of application Ser. No.390,920 filed June 22, 1982, now abandoned. Application Ser. No. 618,712is a division of said application Ser. No. 390,920.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a process and apparatus for makingsolar and semiconductor grade silicon by thermal reaction of a suitableprecursor gas composition. More particularly, the present invention isdirected to a process and apparatus for continuous production of solarand semiconductor grade silicon in the liquid phase, by thermaldecomposition of a suitable precursor gas, such as silane.

2. Description of the Prior Art

As is well known, highly pure elemental silicon properly doped withminute quantities of suitable doping agents, is the most widely usedsemiconductor and solar cell material. In view of the recent trend ofincreasing reliance on solar energy, there exists a significant demandfor solar cell grade silicon at a reasonable cost. In fact, the presentunavailability of solar cell grade silicon at a reasonable costrepresents the principal factor which presently still renders solarcells too expensive for large-scale electrical power generation.

Solar cell or semiconductor grade (hereinafter solar grade) silicon isusually manufactured in a two-step process. First, solid siliconcompounds abundantly available from the Earth's crust (such as SiO₂) areconverted into gaseous or low boiling liquid silicon compounds such assilicon tetrachloride (SiCl₄), trichlorosilane (SiHCl₃) and silane(SiH₄). The gaseous or liquid silicon compounds are then relativelyreadily purified by fractional distillation or like processes.

In the next step of preparing elemental silicon of solar grade purity,the purified silicon compound is reacted in gaseous phase to yieldelemental silicon and usually a gaseous by-product. For example, silanegas is thermally decomposed in accordance with Equation I to yieldsilicon and hydrogen gas. ##STR1##

The above-summarized processes have, hitherto, been performed in theprior art to yield solid elemental silicon. Often the processes yieldvery low overall-density agglomerated particles of silicon, which arehard to handle in an efficient and continuous manner. Other examples ofproblems associated with the gas-to-solid thermal reaction processesare: undesirable deposition of a hard silicon crust on the reactorwalls, and frequent interruption of the process due to the above-notedand other problems. For example, in accordance with the most widely usedprior art modified Siemens process for chemical preparation of solargrade silicon, elemental silicon is grown epitaxially on the surface ofrods disposed in a reactor wherein trichlorosilane (SiHCl₃) and hydrogen(H₂) gases are reacted. However, even this process must be interruptedfrom time to time in order to remove the solid silicon deposited on therods, and to clean the reactor.

Another significant disadvantage of the prior art chemical processes forthe preparation of solid silicon is that the resulting product isusually not sufficiently large grain crystalline to be directly suitablefor semiconductor or solar cell applications. Therefore, the solidsilicon produced by the prior art processes must be melted in a separatestep and converted in a Czochralski or like crystal pulling apparatusinto large grain crystalline (ideally monocrystalline) ingots, ribbonsand the like. Thus, as is well appreciated by those skilled in the art,the overall prior art processes for preparing silicon solar cellsrequire an undesirably high input of energy.

In addition to impurities, the new processes have another formidableproblem; namely, how to limit or control the unwanted gas phaseproduction of submicron silicon particles which are characteristic ofthe thermal decomposition, or pyrolysis, of silicon compounds,especially silane. Two reactions occur in the pyrolysis of a siliconhydride or halide:

(a) homogeneous decomposition reaction to produce fine powder of averageparticle size of about 0.1 micron; and

(b) heterogeneous decomposition on solid surfaces to produce chemicalvapor deposition (CVD) silicon with a metallic appearance.

Fine powder problems have delayed the development of various siliconprocesses using silane.

In order to overcome or alleviate the above-noted problems, a fewattempts were made in the prior art to obtain molten, rather than solid,silicon in the thermal reaction process. For example, Japanese patentapplication laid open for public inspection on Dec. 2, 1977, Ser. No.52-144959, describes a process wherein a bath of molten silicon(obtained from previously-prepared solid silicon of high purity) ismaintained in a reaction vessel wherein trichlorosilane (SiHCl₃) orsilicon tetrachloride (SiCl₄) and hydrogen gas (H₂) are reacted. Thesilicon tetrachloride (SiCl₄) or trichlorosilane (SiHCl₃) is heated to300°-500° C., and the hydrogen gas (H₂) is heated to 1200°-1600° C.prior to introduction into the reaction vessel. The temperature ismaintained in the gas containing part of the reaction vessel between1050° to 1150° C. so that solid elemental silicon is formed in thevessel by the reaction of the gases. The solid silicon, however, fallsinto the bath of molten silicon where it melts.

A readily apparent disadvantage of the just-described process is that itis not suitable for production of silicon from silane (SiH₄), becausesilane would already start significant thermal decomposition while beingpre-heated prior to introduction into the reaction vessel. Furthermore,the reactants used in the process provide elemental silicon only in arelatively low yield. Still further, the process is batchwise, ratherthan continuous, in the sense that the gaseous reactants must be allowedto dwell in the reactor for a relatively long time to reach equilibrium.Perhaps for these and other reasons, according to the best knowledge ofthe present inventor, this prior art process has not gained evenmoderate industrial acceptance.

U.S. patent application Ser. No. 126,063 filed on Feb. 29, 1980, nowU.S. Pat. No. 4,343,772, represents an attempt for production of moltensilicon in a continuously operating reactor by thermal reaction of asuitable silicon containing precursor gas. In accordance with thisdisclosure, a precursor gas, such as silane, flows in an outer,forwardly moving vortex in a spiral flow reactor. A by-product gas, suchas hydrogen, moves in an inner, rearwardly moving vortex. The walls ofthe reactor are maintained at a temperature above the melting point ofsilicon. Molten silicon flows downwardly on the walls of the reactor tocollect in a pool wherefrom it is removed. A cooled injector probehaving an internal diameter of about 0.06 inches is utilized tointroduce the precursor gas tangentially relative to the interiorcylindrical surface of the reactor. A vortex finder tube is disposedsubstantially in the center on the top of the reactor to remove therearwardly moving vortex of the by-product gas.

The reactor described in the above-noted patent application, althoughdesigned to operate continuously for the production of molten silicon,is far from free of problems. More specifically, the injector tube issubject to frequent clogging due to formation of a solid silicon plugtherein, and the emitted by-product gas contains a relatively largequantity of finely dispersed solid silicon particles.

The operation described in U.S. Pat. No. 4,343,772 results in thermalprecipitation of silicon powder onto the probe inside the reactor. Anonmetal solid coating forms thereon and builds outwardly from thecooled probe, getting warmer as it extends. Eventually, the tip of thecoating grows far enough from the probe so that its surface melts andwets. Thereafter, a capillary action phenomenon pulls liquid siliconback toward the cooled probe whereupon more solidifcation occurs,eventually resulting in plugging of the relatively small probe orifice.The significance of conversion of gas to solid prior to melting ofsilicon was unrecognized.

The location of the "vortex finder" near the top of the reactor and nearthe silane injector probe in U.S. Pat. No. 4,343,772, has contributedheavily to the loss of fines by delivering fine brown powder silicon outof the system. Also, the off center entrance of the cold silane probeinto the extremely hot reactor caused large thermal stresses whichresulted in cracking and sealing problems. The process of U.S. Pat. No.4,343,772 limits silicon production because the gas entry and gastake-off are both at the top of the reactor. In cyclones of this design,there is an optimum length for centrifugation, as dictated by enteringconditions and reversal of flow in going from outer vortex to innervortex. Additional length loses the ability to produce additionaleffective cyclonic turns and is non-productive. The velocity of the gasentering the reactor in U.S. Pat. No. 4,343,772 is usually in thehundreds of feet per second.

Additional disclosures relating to the preparation of solar gradesilicon are found in the following printed publications: Low-Cost SolarArray Project 5101-87, "Silicon Formation by Pyrolysis of Silane,"Interim Report of the Continuous Flow Pyrolyzer Study, by H. Levin, JetPropulsion Laboratory, California Institute of Technology, October 1978,and "Compatibility Studies of Various Refractory Materials in Contactwith Molten Silicon," by O'Donnell et al., Jet Propulsion Laboratory,California Institute of Technology, March 1978 (JPL Publication 78-18).

In light of the foregoing, there still is a serious want in the priorart for a continuously operable efficient process and apparatus forchemically preparing molten silicon from a gaseous starting material.The present invention provides such a process and apparatus.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus andprocess for the efficient continuous production of molten silicon from asuitable silicon containing precursor gas.

It is another object of the present invention to provide an apparatusfor the continuous production of molten silicon which readily withstandsprolonged exposure to molten silicon.

These and other objects and advantages are attained by a reactorapparatus wherein a substantially smooth flow of a suitable siliconcontaining precursor gas is maintained in a substantially axialdirection in an elongated reactor heated to a temperature above themelting temperature of elemental silicon. Thermal reaction of theprecursor gas directly yields molten silicon which flows down on thewalls of the reactor to be removed from a bottom thereof in the liquidstate

According to the process of the present invention a septum allows theuse of a large inlet orifice and low velocity entrant flow and therebymore fully utilizes the upper regions of the reactor chamber for siliconconversion. By means of its insulating, high emissivity thermalproperties, the septum eliminates the possibility of brown powderdeposition on a relatively cooler surface, which is the starting step inthe plugging of the inlet orifice.

In known silane decomposition processes, solid submicron silicon isformed and can cause plugging where small orifices and cooled surfacesare present.

In important distinction from the process described in U.S. Pat. No.4,343,772, the present inventive process permits use of a reactor of anylength since no toroidal flow is needed.

In accordance with one aspect of the present invention, an inlet tube ofthe reactor which injects the precursor gas into the reactor isefficiently cooled to prevent thermal formation of silicon prior toentry of the precursor gas into the reactor. One side of a relativelythin septum attached to the inlet tube is substantially in the sametemperature range as the inlet tube, while the other side of the septumis exposed to the interior of the reactor and is substantially in thesame temperature range as the reactor. As a result, the precursor gasentering the reactor is subjected to a very quick transition intemperature with substantially total avoidance of clogging the inlettube by formation of a solid silicon plug.

The precursor gas is admitted to the reactor at comparatively lowvelocity, the very high inlet velocities not being required as set forthor implied in U.S. Pat. No. 4,343,772. A suitable range of velocitiesfor the present invention is 1 to 50 feet per second, preferably 1 to 10feet per second, more preferably 3 to 10 feet per second. Thus for the157 cm by 7.6 cm cylindrical reactor described more fully below, a 1.0cm inlet orifice, for example, may be used rather than the 0.15 cminjector probe exit orifice set forth by U.S. Pat. No. 4,343,772. Thislarger opening is a factor in permitting operation of the reactorwithout clogging. Furthermore, the inlet orifice of the invention may bepositioned at the top center of the reactor, e.g. further from thevertical walls of the reactor. This is of course contrary to thedescription of U.S. Pat. No. 4,343,772, which places the gas outlet tube(the vortex finder) at top center and the gas inlet tube off center andwhich requires a gas inlet flow tangential to the vertical wall of thereactor at a velocity high enough to produce swirl and sustain adouble-vortical flow pattern in the reactor.

The septum of the inventive process is small in size but has a large andcritical role. The septum introduces a precursor gas such as silanewhich begins to change to solid silicon at about 325° C. into a reactorat temperatures above the melting point of silicon without theoccurrence of solid plugging of the gas entry and other untowardeffects. The septum is preferably round and wafer-shaped, with a centralhole and is preferably made of high porosity carbon. For this reason,highly preferred carbon is Union Carbide Corporation's Carbon 60.

One face of the septum device of the invention "sees" very hightemperatures within the reactor where radiation heating controlsdecomposition of precursor gas and melting of silicon. The other face ofthe septum is i contact with the relatively cooled gas inlet tube.Preferably, the septum is in face-to-face contact with the gas inlettube so that the inside diameter holes are matched for smooth gas flow.Thus the septum must be made from a material which has a high emissivitysuch as carbon or graphite. Other high emissivity materials such assilicon carbide may also be used. Emissivity is the relative power of asurface to radiate heat; i.e., the ratio of the energy radiated by asurface compared to a black body perfect radiator (at the sametemperature). The septum of the invention has an emissivity greater than0.90, preferably greater than 0.95. With such a septum, the process ofthe invention may be carried out such that fine brown silicon powdercannot deposit stably on the septum surface. Preferred materials arecarbon and graphite.

The septum of the invention is formed of a material which has a highinsensitivity to thermal shock. The septum or partition, is positionedsuch that one face is in contact with a prodigious heat sink and theopposite face a prodigious heat source. The septum is sufficiently thinthat the gas passes through it rapidly enough so that it does not permitdecomposition of the silane before the silane enters the reactor. Thatis, one face of the septum contacts and covers the flat end face of thegas inlet tube and the other face of the septum preferably sits flushwith the reactor chamber top inner wall and constitutes a part of thetop wall. Only the thickness of the septum separates the gas inlet tubefrom the reaction chamber coaxial with it. In the reactor environmentwhere radiation is the main mode of heat transfer, the emissivity of theseptum face is comparable to that of the reactor walls. The septum mustbe kept of a sufficiently small thickness that silane can pass quicklythrough it without decomposing during the passage. Depending upon thesize of the reactor, the precursor gas, and the flow rate of the gas, asuitable range of thickness for the septum is about 0.1 cm to 1.0 cm.,preferably 0.2 cm to 0.4 cm; and the diameter of the septum hole mayrange from 0.5 cm to 3.0 cm.

In accordance with another aspect of the present invention, the walls ofthe reactor are made of graphite or carbon material which have beendiscovered to be quickly coated (converted) during the initial exposureof the reactor to liquid silicon with a highly stable silicon carbidelayer. Thus, the walls of the reactor, including that portion formed bythe septum, quickly become converted to a material which is vastlysuperior in its ability not to adversely affect the purity of the formedsilicon product.

According to the process of the invention, a decomposable precursor gassuch as silane enters into a preferably vertical, cylindrical reactionchamber at moderate velocity after passing through a cooled gas inlettube and the septum in tandem. The precursor gas, upon entering thereactor, has been heated to a temperature only a few degrees above itsreservoir temperature and well below its decomposition temperature eventhough the reactor is operated at temperatures above the melting pointof silicon.

Where other silicon-containing material such as trichlorosilane, silicontetrafluoride, or silicon tetrachloride in the precursor gascomposition, the composition may be introduced at a higher temperaturewhich is below the decomposition temperature of the silicon-containinggas. Of course, coreactant (reductant) gases, carrier gases and diluentgases may also be used in such compositions. Thus the difference betweenthe temperature of the precursor gas composition at entry and thetemperature of the reaction chamber may be smaller when othersilicon-containing gases are used. For example, SiCl₄ could be presentedto the reaction chamber at 1000° C. without significant gasdecomposition prior to entry into the reaction chamber.

The reactor is preferably coaxial with the gas inlet tube and septum,and preferably incorporates the septum flush with its inner wall at topcenter. Its component and position relationship is highly advantageousto the successful entry of a readily decomposable, solid-producing gasinto an extremely hot reactor without plugging, stress cracking, orotherwise disrupting the reactor operation.

Upon entering the reactor, the precursor material is subjected to a veryquick temperature transition but is maintained in an approximatelyaxi-symmetrical manner of flow down the reactor. The precursor gas heatsup and decomposes to produce chemical vapor deposition (CVD) silicon onthe reactor wall and fine powder silicon in the reactor free space alongwith by-product hydrogen gas or other by-product gas depending upon theprecursor gas. Also, a carrier or diluent gas such as hydrogen may betransferred through the reactor. The CVD silicon and the powder siliconrapidly liquefy under the intense radiant heat within the reactor. Thehydrogen gas and possibly some unreacted precursor gas exit near thebottom of the reactor according to this version of the configuration ofthe reactor and the species of precursor gas. The liquid siliconcollects and is maintained in liquid phase in a reservoir at the bottomof the reactor. The liquid may be drained out of the reactor eithercontinuously or on demand.

In a preferred embodiment, the reactor is a tall-formed,right-cylindrical vessel. It uses carbon or graphite inner walls. Thefirst-made silicon reacts rapidly with the carbon of the inner wall andconverts it to silicon carbide. Thereafter, liquid silicon flows downthe walls and collects in the reservoir during operation. The gas inlettube is seated vertically on the septum and coaxially with the reactorto provide a gas flow vertically down the reactor.

The features of the present invention can be best understood, togetherwith further objects and advantages, by reference to the followingdescription taken in connection with the drawings, wherein like numeralsindicate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the reactor apparatusof the present invention enclosed in an outer cylinder;

FIG. 2 is a schematic cross-sectional view showing the reactor apparatusof the present invention;

FIG. 3 is a schematic perspective view showing a precursor gas inlettube assembly to be incorporated in the reactor apparatus of the presentinvention, and

FIG. 4 is a cross-sectional view showing the precursor gas inlet tubeassembly to be incorporated in the reactor apparatus of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention is a process for directlyproducing molten silicon, said process comprising the

(a) passing a stream comprising silane gas below the decompositiontemperature of silane through a high emissivity, thin, insulating septuminto a noncontaminating reaction chamber above the melting point ofsilicon, said thin septum having one face below the decompositiontemperature of silane and an opposite face exposed to said reactionchamber, whereby said silane gas is decomposed directly to moltensilicon in said reaction chamber;

(b) removing a stream of decomposition hydrogen gas and any unreactedsilane from said reaction chamber; and

(c) recovering product molten silicon from the bottom of said reactionchamber.

A preferred embodiment of the invention is also a process for directlyproducing molten silicon by thermal reaction of a silicon-containingprecursor gas in a reaction chamber, the process comprising the stepsof:

maintaining said silicon-containing precursor in a first temperaturerange below the thermal decomposition temperature of saidsilicon-containing precursor gas;

maintaining the reaction chamber in a second temperaure range above themelting point of silicon;

abruptly raising the temperature of said silicon-containing precursorgas from said first temperature range to said second temperature rangeby introducing said silicon-containing precursor gas into said reactionchamber from a cooled inlet means located outside said reaction chamberthrough a thin, high emissivity, insulating septum having one faceagainst said cooled inlet means and having the opposite face exposed tosaid reaction chamber so that said silicon-containing precursor gasremains undecomposed until entering said reaction chamber;

maintaining a substantially smooth axial flow of said silicon-containingprecursor gas in the reaction chamber while permitting saidsilicon-containing precursor gas to undergo thermal decomposition toyield molten silicon;

removing unreacted precursor gas and by-product gas from the reactionchamber, and

recovering molten silicon from the reaction chamber.

The following specification taken in conjunction with the drawings setsforth the preferred embodiment of the present invention in such a mannerthat any person skilled in the chemical arts can use the invention. Theembodiment of the invention disclosed herein is the best modecontemplated by the inventor for carrying out his invention in acommercial environment, although it should be understood that variousmodifications can be accomplished within the parameters of the presentinvention.

Referring no to the drawing Figures and particularly to the schematiccross-sectional views of FIGS. 1 and 2, the reactor apparatus 10 of thepresent invention is disclosed. The reactor apparatus 10 of the presentinvention is adapted for producing high purity molten silicon in acontinuous process by thermal reaction of a suitable silicon containingprecursor gas composition. Several known chemical reactions may beutilized in the novel process practiced in the reactor apparatus 10. Forexample, silicon may be produced in the apparatus 10 by reduction ofsilicon tetrachloride (SiCl₄) or trichlorosilane (SiHCl₃) with hydrogen(H₂) in accordance with Equations II and III respectively. ##STR2##

However, the apparatus 10 and process of the present invention isdesigned most advantageously for continuous production of silicon bythermal decomposition of silane gas into silicon and hydrogen gas inaccordance with Equation I (set forth above). Therefore, the followingexemplary description principally describes the utilization of thethermal decomposition reaction of silane in the novel apparatus andprocess of the present invention. Furthermore, hereinafter the term"precursor gas" is interchangeably used with the term "silane," and theterm "by-product gas" is interchangeably used with the term "hydrogen."Nevertheless, it should be kept in mind that in the event the process ofthe present invention utilizes alternative chemical reactions (such asthe reactions of Equations II or III), the precursor gas may be otherthan silane, and the by-product gas or gases may be other than hydrogen.

Referring now principally to FIG. 1, the reactor apparatus 10 of thepresent invention is shown mounted in a suitable outer cylinder 12. Theprincipal purpose of the outer cylinder 12 is to contain the reactorapparatus 10 in an inert gas atmosphere while the reactor apparatus 10is heated to high temperatures. The outer cylinder 12 may be constructedsubstantially in accordance with the state-of-the-art. The outercylinder 12, of course, also provides thermal insulation for the reactorapparatus 10, and for that purpose walls 14 are made of stainless steelencasing suitable insulating materials (not shown) which surround thereactor apparatus 10. Inlet and outlet tubes for the inert gas, which ispreferably argon, are schematically shown on FIG. 1 and bear thereference numerals 16 and 18, respectively.

The protective outer cylinder 12 also has suitable inlet ports for ductscarrying the precursor gas, the by-product gas, and the reactionproduct, molten silicon 20. On the schematic view of FIG. 1, the inletduct assembly for the precursor gas silane bears the reference numeral22, the outlet duct for the by-product hydrogen the reference numeral24, and the outlet duct for molten silicon the reference numeral 26.Finally, the outer cylinder 12 is provided with suitable entry ports forcopper or like conduits 28 which conduct current for energizing graphite"picket fence" type resistance heaters 30 surrounding the inner reactorapparatus 10. The resistance heaters 30 and the inner reactor 32 areseparated from one another by a high-temperature-resistant electricalinsulator 31.

Referring now principally to the schematic cross-sectional view of FIG.2, the inner reactor apparatus 10 is disclosed in detail. The reactorapparatus is an elongated hollow body; preferably, as is shown in theherein-described preferred embodiment, it is a hollow cylindrical bodythe length of which greatly exceeds its diameter. In a specific exampleof the reactor apparatus 10 of the present invention, the cylindricalreactor body or reactor 32 is 157 cm long, and has an inner diameter of7.6 cm. The length-to-width ratio of the reactor may vary over a widerange so long as molten silicon is produced and decomposition gas isremoved. A suitable range is at least about 2:1 or higher. The relativelength-to-width ratio of the reactor 32 is designed to provide asufficiently long dwelling time for the gaseous reactants in the reactor32 to reach thermodynamic equilibrium, which, under the conditionsprevailing in the reactor 32, favors high yields of elemental silicon.In fact, in the apparatus of the present invention, silane gas isconverted in substantially one hundred percent (100%) yield to silicon.

In the cyclone apparatus of U.S. Pat. No. 4,343,772 the length-to-widthratio is limited to optimum length dictated by reverse flowconsiderations. Additional length therein is ineffective andnonproductive. The throughput of the present invention is not solimited.

In accordance with one important aspect of the invention, the reactor 32is made of highly pure graphite or carbon material which rapidly reactsunder the conditions prevailing in the reactor 32 with silicon to formsilicon carbide (SiC). In fact, it was found in experience that thesilicon formed in the initial stages of the operation of the reactorapparatus 10 is substantially consumed to react with the inner walls ofthe reactor 32. The resulting silicon carbide coating on the reactorwalls is highly wettable by molten silicon, and is yet highly resistantto molten silicon. This is a very advantageous feature of the reactorapparatus 10 of the present invention, because it permits the formationof highly pure solar grade silicon substantially uncontaminated bymaterial dissolved from the reactor walls.

For comparison, it is noted that liquid silicon is known in the art tobe highly reactive and is often termed a "universal solvent." In fact,most prior art silicon crystal pulling apparatus struggle with theproblem of keeping molten silicon sufficiently free of impurities formedwhen the liquid silicon reacts with, dissolves, or diffuses impuritiesout from the walls of a crucible, capillary die, or the like. Thepresent invention, however, completely solves this problem by providingthe readily available graphite or carbon reactor material, which duringthe process of the invention "self-converts" into the highly durable andliquid-silicon-resistant silicon carbide. The initial stage ofconverting the carbon walls of the reactor 32 into silicon carbide istermed "priming" of the reactor.

Heating of the reactor body 32 is accomplished by the "picket fence"type resistant heaters 30 which surround the reactor body 32. Inalternative embodiments of the reactor apparatus 10 of the presentinvention, other methods of heating may be utilized. The only criticalfeature of the invention regarding heating is that the interior of thereactor 32 and the interior walls are maintained above the meltingtemperature of silicon (1412° C.) during the process of siliconproduction. Preferably, the interior of the reactor 32 is maintainedabove 1500° C., and most preferably it is maintained approximatelybetween 1600° to 1800° C. At the operating temperatures of the reactorapparatus 10 (i.e., above 1412° C.), the thermal decomposition of silanegas into silicon and hydrogen is extremely rapid. Furthermore, at thistemperature the thermodynamic equilibrium favors formation of elementalsilicon with substantially one hundred percent (100%) yield.

Actually, the above-noted upper limit of approximately 1800° C. is not alimit of the process of the present invention, because the process maybe practiced at still higher temperatures. Rather, approximately 1800°C. is the upper limit of temperature that the materials of the reactorapparatus 10 of the present invention can withstand without damage inindefinite continuous operation.

With regard to the above-noted temperature ranges of the process of thepresent invention, it is further noted that conversion of the carbon ographite walls of the reactor 32 to silicon carbide occurs below 1300°C. too slowly to have any practical effect on priming the reactor 32.Therefore, in the prior art reactors operating at temperatures below1300° C., the very advantageous "priming" of the reactor does not occurto a useful extent.

Experience proved the reactor apparatus 10 of the present invention tobe capable of withstanding repeated temperature cycling between ambienttemperature (nonoperational time of the reactor) and 1400°-1800° C.(operation).

CVD silicon is formed on the walls of reactor 32 and fine silicon powderis formed in the free space within the reactor. Residence time withinthe reactor environment is sufficient to melt the fine powder siliconand transport it to the walls or directly to the reservoir of liquidsilicon at the bottom of the reactor 32. In very short versions of thereactor, the melting of the powder may be aided by a transverse filteror a baffle intercepting the powder stream within the reactor so thatlittle or no silicon powder can escape the reactor.

The silicon formed in the reactor 32 is in the molten phase, and flowsdown the silicon carbide interior walls of the reactor 32 to collect inthe bottom thereof. In a preferred operation, pure, solar grade moltensilicon is drained from the bottom of the reactor 32 and isadvantageously utilized, without being allowed to solidify, in aCzochralski or other crystal shaping equipment (not shown) to providesilicon ingots or ribbons (not shown).

Preferably, the reactor apparatus 10 is disposed in a substantiallyupright position, as is shown on FIGS. 1 and 2, so that flow of themolten silicon 20 on the reactor walls is gravity induced. The reactoris preferably operated with a smooth axial flow of the precursor gascomposition from the top toward the bottom of the reactor.Alternatively, the gas could be flowed from the bottom toward the top ofthe reactor as molten silicon flows downwardly to collect at the bottomof the reactor.

Referring now principally to FIGS. 3 and 4, the silane gas inlet duct ortube assembly 22 is disclosed in detail. The inlet tube assembly 22 isparticularly adapted and highly suited for supplying silane gas into thereactor 32 in a continuous and substantially nonclogging manner. As isshown on FIG. 2, the inlet tube assembly 22 is disposed substantially inthe center of a top wall 34 of the reactor 32 in such a manner that theinlet tube assembly 22 is located outside of the heated interior spaceof the reactor 32.

The inlet tube assembly 22 comprises an elongated inlet tube 36 which isconnected to a supply of silane gas. The inlet tube 36 is surrounded bya jacket 38 wherethrough a cooling medium, such as water, is circulated.The end of the inlet tube 36 and the end of the jacket 38 together forma substantially flat substantially ring-shaped surface 40.

The ring-shaped surface 40 of the inlet tube assembly 22 is not directlyexposed to the interior of the reactor 32. Rather, it is in contact withone side 42 of a carbon or graphite septum 44. The septum 44 comprises arelatively thin ring-shaped body which substantially conforms to theshape of the end of the inlet tube assembly 22. A second side 46 of theseptum 44 is substantially flush with the interior surface of the topwall 34 of the reactor 32, as is shown on FIG. 2.

The septum 44 insulates the inlet tube 3 from the high temperature ofthe reactor 32 and causes the silane gas which enters the reactor 32 toexperience a very sharp temperature gradient. This is because one side42 of the septum 44 is in contact with the efficiently cooled surface 40of the inlet tube 36 and jacket 38, and is substantially in the sametemperature range as the inlet tube 36. The other side 46 of the septum44 is exposed to the heated interior of the reactor 32, and, being of adark high emissivity material (porous carbon), is substantially in thesame temperature range as the interior of the reactor 32.

During the process of the present invention, silane gas is continuouslyfed through the inlet tube assembly 22 into the reactor 32. The inlettube assembly 22 is vigorously cooled by water circulated through thecooling jacket 38. In accordance with one important aspect of thepresent invention, the temperature of the silane gas introduced into thereactor 32 through the inlet tube 36 is kept at a temperature below 300°C. This is for the purpose of avoiding any significant thermaldecomposition of the silane gas prematurely, before entry of the silanegas into the reactor 32. In order to facilitate efficient cooling of theinlet tube assembly 22, the inlet tube 36 and the jacket 38 are made ofa material of high thermal conductivity, such as copper.

A wide range of smooth, laminar flow rates are usable according to theprocess of the invention. The flow rate should be at least enough toprovide a reasonable throughput for the reactor and not so high as tocause cool zones or cool portions of reactor wall. Also the flow rateshould not be so high as to interfere with removal of molten siliconfrom the bottom of the reactor and withdrawal of decomposition and othergases. A suitable flow rate for the gas is at least 0.02 cubic feet perminute (cfm) (0.6 liters gas per minute) STP up to a maximum flow rate,depending on the size of the reactor, wherein the operation of thereactor is not affected. A suitable range of linear gas velocity isabout 1.0 to 50 feet per second (fps) most preferably 3 to 10 fps.

The relatively thin septum 44 of carbon or graphite is highly effectivein avoiding thermal precipitation of fine solid silicon particles on theinlet surfaces and ensuing clogging of the inlet tube and the narrowseptum 44 itself. Experience has shown that during the "priming" stageof the operation of the reactor apparatus 10, the septum 44 too reactswith the newly-formed liquid silicon to form silicon carbide, withoutimpairing its emissivity.

The inlet tube assembly 22 of the reactor apparatus of the presentinvention operates without clogging. In contrast with the prior artinlet assembly disclosed in the above-noted U.S. patent application Ser.No. 126,063, now U.S. Pat. No. 4,343,772 the internal diameter of theinlet tube 36 may be quite substantial. In the herein-described specificexample wherein the length of the reactor is 157 cm, the internaldiameter of the inlet tube 36 is approximately 1 cm, and the insidediameter of the abutting septum 44 is approximately 1 cm. The outsidediameter of the septum 44 is 2.2 cm and its thickness is about 0.3 cm.

Dwelling time of the silane gas in the heated reactor 32 is selected insuch a manner that the silane-to-liquid silicon reaction proceeds tosubstantially reach thermodynamic equilibrium which favors completedecomposition. During the process, the silane gas and the by-producthydrogen gas flow in a substantially smooth, substantially unperturbedflow, substantially axially in the reactor 32. The lower, reservoirportion of the reactor 32, containing molten silicon 20, is kept at atemperature exceeding the melting temperature of silicon. The moltensilicon may be continuously drained out of the reactor apparatus 10through the outlet duct assembly 26 which may be constructed inaccordance with state of the art. The by-product hydrogen gas leaves thereactor 32 through the hydrogen outlet duct assembly 24 which is locatedon a side wall of the reactor 32 above the level of molten silicon 20.Assembly 24 is located near the bottom of the reactor 32 (above themaximum level of molten silicon 20) so that gas flow is essentially fromtop to bottom. Alternatively, the gas outlet tube may extend upward fromthe bottom of the reactor through the pool of molten silicon.

As indicated above, in one aspect this invention comprises thepreparation of highly pure silicon by operation of the above-describedreactor apparatus using a suitable silicon-containing precursor materialother than silane. Such other silicon-containing precursor materials areexemplified by trichlorosilane, silicon tetrachloride and silicontetrafluoride. Thus, such other materials contain halogen, or hydrogenand halogen bonded to silicon. As indicated by the suitability of bothfluorine and chlorine compounds, the nature of the halogen bonded tosilicon in the precursor materials is not critical. The compounds maycontain other chemical species such as oxygen, in addition to hydrogenand halides. Furthermore, the halogen atoms may be alike, as indicatedabove; or alternatively, they may be different. Thus, for example, inprecursor materials suitable for this invention, fluorine and chlorineboth may be present in the same molecule bonded to silicon. If desired,the halogen-containing precursor materials can be admixed, or admixedwith silane. Also, one precursor material can be used followed by use ofanother.

The preferred halogen-containing compounds utilized as precursormaterials are available at a suitable cost. In this regard however, itis to be understood that the cost of a precursor material is animportant, but not a critical, requirement of this invention.

A skilled practitioner will recognize a relationship between processtemperature and precursor material employed as a gas. The process ofthis invention necessarily entails operation at temperatures above themelting point of silicon (1412° C.). Furthermore, as pointed out above,experience proved the reactor apparatus of the present invention iscapable of withstanding repeated temperature cycling between ambienttemperature (nonoperational time of the reactor) and 1400°-1800° C.Within these parameters, it is generally desirable to conduct theprocess of this invention at a temperature or temperatures at whichthermodynamics favor thermochemical decomposition of the selectedprecursor gas or gases to essentially pure silicon.

Silicon-containing precursor materials other than silane can be usedanalogously to silane. Thus, the halosilane precursor used in thisinvention can be introduced with hydrogen via cooled tube inlet meansthrough a septum having a thickness of about 0.1 to 1.0 centimetersthick, and a matching orifice of a diameter in the range of 0.5-3.0 cm.The emissivity of the spectrum can be 0.90 or higher. Preferably, thefirst temperature range is below 200° C., more preferably below 1000° C.and the second temperature inside the reactor is approximately1500°-1800° C., more preferably approximately 1600°-1800° C. The septumlocation is preferably at the top center of the reactor and the feed gasis preferably introduced at a rate of about 1.0 to 50.0 feed per second,more preferably at about 3.0 to 10.0 feet per second.

COMPARATIVE EXAMPLE

Four runs were made in which silane gas was fed out of room temperaturestorage through a cooled inlet tube and into a reactor, as described inU.S. Pat. No. 4,343,772. In all four runs, the reactor temperature wasbetween 1600° and 1700° C. In the first three runs, the entry gas was100% silane flowing at 0.1 cfm STP; in the fourth run, the gas was 20%silane, 80% hydrogen, at 0.25 cfm STP. Accordingly, gas dischargevelocity from the inlet tube into the reactor varied from about 100 to250 fps. Within the reactor, the positions of inlet tube and vortexfinder were varied over the four runs and included the preferredpositions of U.S. Pat. No. 4,343,772.

In each run, operation was terminated after 5 minutes due to excessiveinlet gas pressure caused by complete or almost complete plugging of theexit of the inlet tube, as determined by pressure increase and confirmedlater by visual examination. The inlet tube exit tip was seen coveredwith a plug, which was a gray, poorly consolidated mass of silicon,typically about 4 millimeters in length extending from the tip andweighing about 1 gram. Furthermore, the terminated runs were found to benear to failure due to a second cause: accumulation of a fine yellowishbrown silicon powder in the vortex finder at its first constriction justabove the reactor. Also, the top of the reactor, upon cooling, showedincipient cracks radiating from the off-center inlet tube.

In all the runs, there was no evidence of silicon collection at thebottom of the reactor, but there was some indication of conversion ofgraphite to silicon carbide at the reactor wall near the vortex finder,as shown by increased abrasion resistance. In the fourth run, brownsilicon powder was recovered from the exit lines; and a mass balance wastaken, which indicated that all of the silane had been decomposed andmost of it converted to the brown silicon powder.

The loss of silicon powder was attributed to the location of the vortexfinder near the top of the reactor where silicon particles initiallyform. At this location, the vortex finder can have a large tendency tosweep out freshly formed submicron powder entrained in the gas stream.

EXAMPLE

A gas composed of 20% silane and 80% hydrogen was provided to a verticalgraphite reactor as described above for the invention and shown in thedrawing figures, except that the reactor chamber was abbreviated tolength 21 cm by 5 cm internal diameter, and except that a 1.9 cm-IDoutlet tube extended axially downward out of the reactor from an openend 2 inches above the chamber bottom (to allow a pool of molten siliconto form at the chamber bottom and at the same time to allow theaxisymmetric downward flow and discharge of gases and entrained powderthrough the outlet tube).

The silane/hydrogen mixture flowed into the reactor chamber from roomtemperature storage through a cooled, vertical inlet tube and through a0.3 cm-thick porous carbon septum. The inlet tube had a 1.0 cm diametergas flow channel, and the septum had a 1.0 cm central hole diameter--avery significant difference from the 0.16 cm-diameter of the aperture ofthe inlet tube of U.S. Pat. No. 4,343,772. One face of the septumconcentrically abutted the flat end face of the inlet tube. The otherface of the septum was central and flush with the top wall of thereactor. Emissivities of both the septum and the reactor wall were wellabove 0.9. The silane/hydrogen mixture flowed through inlet tube andseptum at 0.25 cfm STP (at a velocity of about 5 fps) and entered thereaction chamber, which was heated by an external graphite resistanceheater to about 1650° C. In the reactor, the silane decomposedcompletely (as evidenced by the color and luminosity of a burn-off flameindicator at the exit of the apparatus); and hydrogen and entrainedsilicon powder entered the outlet tube and exited the bottom of thereactor.

The run was discontinued after 20 minutes because of increased pressuredownstream of the reactor due to deposition of powdered silicon in thelines. This escape of most of the powdered silicon from the reactor wasto be expected since the test reactor chamber was only 21 cm long, andthe effective reaction length was only 15 cm--a small segment actuallyof the preferred embodiment, which has been given as 157 cm in length by7.5 cm ID. At full design length (157 cm, compared to 21 cm) no powderedsilicon is expected to escape. In essence, the dwell time in theabbreviated reactor, about one second, was insufficient to accomplishcompletely the sequential steps of silane heating, pyrolysis, siliconpowder generation, nucleation, particle growth, heating and melting.Nevertheless, the weight of the reactor increased by 10.1 grams due toin situ conversion of the graphite wall to silicon carbide, showing that"priming" of the reactor for liquid silicon collection was readilytaking place.

The reactor was cooled, disassembled and inspected. The septum wasclear, and there was no evidence of silicon powder deposit anywhere inthe reactor. This 20-minute run conclusively demonstrated a plug-freeinlet operation.

Following the general procedure of the above example, a high-puritysilicon can also be obtained using the apparatus of this invention andother silicon-containing materials such as the halosilanes and thehalosilicons, as exemplified by trichlorosilane, silicon tetrachlorideand silicon tetrafluoride. The silicon-containing material can be heatedto a temperature of 300°-500° C. or higher, and hydrogen to 1200°-1600°C. or higher in order that the resulting composition be gaseous prior tointroduction into the reaction vessel.

It is thus seen that no change in length, shape, size, or operatingconditions of the cyclone-type reactor of U.S. Pat. No. 4,343,772 canprevent the loss of submicron silicon powder such as we have due to thevortex finder influence of the gas stream in the prior art device. Incomparison, the reactor of the invention can be operated with adequatedwell time of the silane gas by appropriately sizing the reactor toavoid any loss of submicron silicon powder by completely melting thesilicon.

Principal advantages of the above-described reactor apparatus andprocess of the present invention include the following. The reactor 32is readily manufactured of relatively inexpensive pure carbon orgraphite material, and yet readily "primes" or converts itself toeventually expose only non-contaminating silicon carbide for contactwith the highly pure, solar grade silicon. The reactor is capable ofcontinuous, trouble-free operation, is not subject to clogging due tobuildup of solid silicon material, and does not require continuousscraping or clearing operation to keep the walls free of deposited hardcrust of silicon.

The above-described specific embodiment of the reactor apparatus 10 iscapable of producing several kilograms (up to 15 kg) of solar grademolten silicon per hour. For a still larger scale silicon producingplant a still larger embodiment of the reactor apparatus 10 of thepresent invention may be utilized. Alternatively, and preferably,several reactors of approximately of the above-noted specific dimensionsmay be operated simultaneously.

Furthermore, and preferably in small versions of the reactor, the moltensilicon chemically obtained in the process of the present invention maybe directly fed into a Czochralski or other crystal shaping apparatus(not shown), eliminating the need for remelting of solid silicon. Theenergy saving thereby is considerable. Alternatively, the molten siliconmay be used to produce castings for later remelt and replenishment ofCzochralski or other apparatus.

This invention can be extended to the preparation of silicon carbidecoatings of carbon or graphite objects or, alternatively to thepreparation of silicon carbide articles of manufacture from carbon orgraphite preform objects.

As indicated above, the reactor apparatus of this invention is made ofhighly pure graphite or carbon material which rapidly reacts under theconditions prevailing in reactor 32 with silicon to form siliconcarbide. In fact, as mentioned above, it was found in experience thatthe silicon formed in the initial stages of the operation of reactorapparatus 10 is substantially consumed by reacting with the inner wallsof reactor 32.

In order to prepare silicon carbide articles of manufacture in anembodiment of this invention, advantage is taken of the above-mentionedtendency of silicon to react with the reactor walls. To do so, onemounts or places one or more carbon or graphite objects, preferably ofcomparatively small size, within the chamber of reactor 32 and thenconducts the process of this invention as above described, using asuitable silicon-containing precursor material to produce silicon insidereactor 32. During operation in this manner, silicon can contact theobject or objects to produce a coating of silicon carbide for abrasionresistance which is highly desirable in nozzle throat applications.Alternatively, by purposeful variation of contact time, the silicon canconvert the object or objects in depth partially or entirely to siliconcarbide. A carbon core may remain, particularly if the object isrelatively dense or thick. Additional excess silicon will wet thesilicon carbide and be absorbed within the silicon carbide (or siliconcarbide/carbon core) body. After converting the objects to the degreedesired, the treatment process is stopped and silicon carbide-coatedobjects, or silicon carbide objects, or silicon carbide/silicon objectsor silicon carbide/carbon core objects, or siliconcarbide/silicon/carbon core objects are removed from the reactor. Whenexcess or free silicon is desired to remain (as in siliconcarbide/silicon objects), it provides the important advantage of fillingthe voids and forming a non-porous body.

For this embodiment, one can make modifications in the reactor apparatusto provide for high object-holding capacity and to allow more easyaccess to the interior of the reactor, thereby facilitating adding moreparts to be coated by repeating the process described above. Forexample, outer cylinder 12 can be hinged to allow an operator to open aside of 12 and reach inner reactor 10. Suitable modifications ofresistance heaters 30, insulator 31, and inner reactor 32 are also madeto allow easy access to the interior of 32. Also, for example, heaters30, insulator 31, and inner reactor 32 can be segmented above and belowconduits 28, and the segmented parts "stacked" one on top of the otherto provide the elongated hollow body shown in FIGS. 1 and 2. Inaddition, mounting stations for the objects to be coated can be added tothe interior wall of 32.

It is to be understood that the batch process described above, providesobjects that are coated with or converted to silicon carbide and siliconwith little or no increase in dimension(s) of the object. This is animportant feature of the invention, and is especially important whenpreparing objects for use in the electronic industries.

As an alternate to the batch process described above, automation of theloading, converting, and removing steps can be contemplated to takeadvantage of the continuous production of silicon carbide coatings orbodies provided by operation of the reactor apparatus of this invention.

Several modifications of the above-described apparatus and process maybecome readily apparent to those skilled in the art in light of theabove disclosure. Therefore, the scope of the present invention shouldbe interpreted solely from the following claims.

I claim:
 1. A process for coating a carbon or graphite object withsilicon carbide by contacting it for a period of time with siliconliquid and vapor in a reaction chamber, said process comprising thesteps of:(a) passing a stream comprising silicon-containing precursormaterial from the group consisting of silane halosilanes andhalosilicons in gaseous phase below the decomposition temperature ofsaid material and a co-reactant, carrier or diluent gas such as hydrogenthrough a hole within a high emissivity, thin, insulating septum intosaid reaction chamber; said reaction chamber being maintained at atemperature above the melting point of silicon, said temperature beingat least 1400° C.; said thin septum having one face below thedecomposition temperature of said gas and an opposite face exposed tothe reaction chamber, whereby said precursor gas is decomposed tosilicon liquid in said reaction chamber; (b) removing a steam of anydecomposition gas and any unreacted precursor gas from said reactionchamber; and (c) contacting said object in said reaction chamber withsaid silicon liquid, and recovering said object from said reactionchamber after it has been coated with silicon carbide.
 2. The processaccording to claim 1 in which said temperature is maintained above 1500°C.
 3. The process according to claim 1 in which said temperature is inthe range of 1500° C. to 1800° C.
 4. The process according to claim 3 inwhich said halosilane is trichlorosilane.
 5. The process according toclaim 3 in which said halosilicons are selected from the groupconsisting of silicon tetrafluoride and silicon tetrachloride.